This invention relates to ablative materials for use in provided thermal protection to reentry vehicles. More particularly, this invention concerns itself with the fabrication of ablative materials having reduced plasma electron density.
The problem of providing thermal protection for reentty vehicles has become of paramount importance with the recent interest in space travel. The reentry vehicle often contains a number of scientific and communication instruments programmed to send back various technical data to ground control stations. These instruments are particularly vulnerable to damage at the elevated temperatures encountered during the reentry regime. Consequently, a large research effort has evolved in attempting to overcome the reentry heat problem. Various ceramic materials have been suggested as ablative shields because of their excellent heat resistance. These materials, however, do not compensate for the difference in thermal expansion between the ceramic shield and the material of the reentry vehicles. The interval stress generated by the difference in thermal expansion produces cracking which minimizes the insulation value of the ceramic.
In order to overcome the cracking problem, a number of resinous material has been evaluated for their heat resistance. A carbon-phenolic laminate, for example, gives a high degree of thermal protection with little evidence of cracking during the stress and strain of reentry. However, the carbon-phenolics produce a very high density of electrons which contributes to a communications blackout under same conditions between the reentry vehicle and its ground control station.
With the present invention, however, it has been found that certain materials are capable of decreasing the electron density in the reentry plasma sheath when they are added to the sheath by spraying out from the surface. The plasma of the ablative material of this invention greatly reduces electron density and tends to eliminate the communications blackout often encountered during reentry. The material of the invention is composed of a phenolic resin impregnated carbon or graphite cloth laminated with a polytetrafluoroethylene resin.